Abstract: In order to study the deformation behavior of a thermoplastic olefin (TPO) during tensile loading, a constitutive model based on molecular chain evolution was developed and was implemented into a finite element method (FEM). Then the deformation behavior and microstructure were coupled. The model includes rate dependent initial uniform deformation, yielding, strain softening, and strain hardening. The FEM was used to simulate the deformation behavior of TPO during tensile test. The results show that the constitutive model results are very close to the experimental data. The simulation results give a clear description of stress distribution and evolution of necking during threedimensional deformation. The FEM results provide fruitful information for virtual design of TPO parts under development.